Non-monocrystalline emitters, e.g., of polysilicon, have the disadvantage that their doping usually requires an implantation step or an in-situ doping step during the deposition, as well as a subsequent tempering process, in order to drive the dopant into a monocrystalline region. The disadvantages are caused by the significant heat supply into the transistor layer system that leads to a widening of the intrinsic (active) base, to an increased point defect diffusion and similar effects. This results in a widened base profile and limits the maximum attainable switching speed of the transistor that is defined by the transit time required by the charge carriers in order to cross the base. In addition, a highly doped emitter-base junction with a high emitter-base capacitance and a low breakdown voltage of the junction is achieved in this fashion. However, the polycrystalline structure of the emitter also manifests itself in the surface and affects the smoothness of the emitter-base junction due to its geometric structure, wherein diffusion effects of the dopant used for the emitter are observed, in particular, on the grain boundaries.
Two different methods are generally employed for manufacturing a bipolar transistor with an epitaxial base and an epitaxial emitter layer. According to the first method, a base layer is grown epitaxially and an insulating layer is subsequently produced thereon and structured in order to expose the intrinsic base. The emitter layer can then be epitaxially deposited thereon. The disadvantage of this method can be seen in that the time-consuming epitaxy needs to be carried out in an epitaxy reactor and interrupted after the base layer is produced. Once the insulating layer is produced and structured accordingly, the wafer needs to be placed back into the reactor and the epitaxy conditions need to be restored. This additionally increases the expenditure of time. When the window in the insulating layer is opened, the unprotected intrinsic base is also subjected to the etching medium used. This may result in damages to the surface structure or the doping of the base.
In the second method, the base layer consists of a silicon-germanium alloy and is grown epitaxially. If a silicon layer is epitaxially grown directly thereon, the silicon-germanium alloy of the base layer may serve as an etching barrier during the subsequent structuring of the emitter layer. The disadvantage of this method can be seen in that the surface of the base layer needs to have a high germanium content of approximately 20% in order to achieve a high etching selectivity relative to silicon. However, it was determined that such high germanium concentrations at the base-emitter junction are not advantageous with respect to high switching speeds. Despite the high etching selectivity relative to silicon, the base layer may become damaged during the structuring of the emitter layer in the exposed surface regions of the base.